Recently, a secondary battery, which can be charged and discharged, has been widely used as an energy source for wireless mobile devices. Also, the secondary battery has attracted considerable attention as a power source for electric vehicles and hybrid electric vehicles, which have been developed to solve problems, such as air pollution, caused by existing gasoline and diesel vehicles using fossil fuel. As a result, kinds of applications using the secondary battery are being increased owing to advantages of the secondary battery, and hereafter the secondary battery is expected to be applied to more applications and products than now.
As kinds of applications and products, to which the secondary battery is applicable, are increased, kinds of batteries are also increased such that the batteries can provide outputs and capacities corresponding to the various applications and products. Furthermore, there is a strong need to reduce the sizes and weights of the batteries applied to the corresponding applications and products.
For example, small-sized mobile devices, such as mobile phones, personal digital assistants (PDAs), digital cameras, and laptop computers, use one or several small-sized, light cells for each device according to the reduction in size and weight of the corresponding products. On the other hand, medium- or large-sized devices, such as electric bicycles, electric vehicles, and hybrid electric vehicles, use a battery module (or battery pack) having a plurality of cells electrically connected with each other because high output and large capacity is necessary for the medium- or large-sized devices. The size and weight of the battery module is directly related to the receiving space and output of the corresponding medium- or large-sized device. For this reason, manufacturers are trying to manufacture small-sized, light battery modules. Furthermore, devices, which are subject to a large number of external impacts and vibrations, such as electric bicycles and electric vehicles, require stable electrical connection and physical connection between components constituting the battery module. In addition, a plurality of cells are used to accomplish high output and large capacity, and therefore, the safety of the battery module is regarded as important. Consequently, the battery module includes switching elements for controlling overcharge, overdischarge, and overcurrent and a circuit for controlling the switching elements.
Also, as there are many kinds of products using the secondary battery in medium- or large-sized device applications, as previously described, various battery modules that are capable of providing corresponding electrical capacity and output are necessary. Furthermore, when the sizes of products are different even in the same product group, the corresponding electrical capacities and outputs are also different, and therefore, it is necessary to change the design of the battery module.
The conventional medium- or large-sized secondary battery module is constructed in a structure in which a plurality of unit cells are received in a case (housing) having a predetermined size, and the unit cells are electrically connected with each other. At the inside or the outside of the case are mounted circuit elements for sensing the voltage, the current, and the temperature of the unit cells and controlling the battery. However, this structure of the battery module, especially, the elements for controlling overcharge, overdischarge, and overcurrent, have several problems.
The switching elements used to control overcharge generate a large amount of heat. For this reason, the switching elements are generally mounted at the outside of the battery module while the switching elements are connected to an additional heat dissipation structure, and therefore, it is difficult to appropriately modify the heat dissipation structure when the size of the battery module is to be increased or decreased depending upon desired electrical capacity and output. Also, the size of the heat dissipation structure is an obstacle to design of a compact battery module.